This invention relates to an image forming apparatus having a toner density detecting device.
Conventionally, in an image forming apparatus using bicomponent developer consisting of toner and carrier, a developing unit is provided with a toner density sensor for detecting toner density of the developer. Supply of the toner is controlled in accordance with a sensor signal from the toner density sensor to maintain a proper toner density, thereby preventing the toner from being excessively or deficiently fed to a photosensitive drum.
A low cost image forming apparatus requires assemblage from low cost parts. However, toner density sensors are generally relatively expensive because they include a magnetic sensor or like sensor. Accordingly, there has been an increasing demand for a toner density sensor which can be fabricated at low cost, instead of the existing expensive toner density sensors.
It is a well-known concept that a resistance value of developer varies according to a mixing ratio of toner to carrier, namely toner density, in the developer.
In view of the above concept, Unexamined Japanese Patent Publication 58-169161 has proposed an image forming apparatus including a detector circuit in which a resistor is connected between a regulating member for regulating thickness of a layer of developer formed on the surface of a developing roller and a ground to detect the toner density. More specifically, a bias voltage is applied to the developing roller in order to prevent formation of fog on a copy. The detector circuit detects an electric current caused by the bias voltage and flowing into the resistor through the developing roller, developer layer and regulating member, whereby to detect the toner density.
However, in the above detector circuit, the bias voltage is divided by the resistance of the developer and the resistor. The surface voltage of the developer layer formed on the developer roller is reduced, thereby making it difficult to reliably prevent formation of the fog on the copy.
In order to avoid the above drawback, it may be considered to increase the level of the bias voltage. However, this necessitates fabrication of the developing roller having improved resistance against voltage at an increased cost.